In recent years, the output power of fiber lasers continues to increase, and fiber lasers with high output power of more than a kW-order have been developed. Such high power fiber lasers are used in various industrial applications such as processing machines, medical apparatuses, and measuring instruments. The use of fiber lasers have been rapidly widened because in the material-processing field, fiber lasers are superior to other lasers in their light focusing properties and can significantly increase the power density at focusing points, so as to enable precision machining, and because it enables performing a noncontact processing, and also enables processing of hard materials which can absorb laser light.
FIG. 1 shows a schematic view of a typical high power fiber laser which is called a master oscillator—power amplifier (MOPA) type. The MOPA type fiber laser is configured to connect the rear stage of the MO with the PA, and obtains the high power laser light by amplifying the weak pulse light output from the MO using the PA. When sufficient power cannot be obtained by a PA of one stage, PAs are connected in a multistage manner to obtain the desired power.
For the Mo, there is a type in which the density of the output power of the laser light source which performs CW oscillation such as a semiconductor laser, is intensity-modulated using a modulator such as an acousto-optic element in order to obtain pulse light. There is also a type for the MO, which employs a fiber ring laser, as shown in Patent Document 1, for example. FIG. 2 is a view illustrating a structure of a typical optical fiber ring laser.
The known fiber ring laser 310 shown in FIG. 2 is composed of a pump light source 311, a WDM coupler 312 which combines pump light and laser light, a rare earth-doped optical fiber 313 which is a gain medium, an isolator 314, a light switch element 317, and an output coupler 315. The pump light emitted from the pump light source 311 is coupled to the rare earth-doped optical fiber 313 via the WDM coupler 312. The pump light coupled to the rare earth-doped optical fiber 313 is absorbed into rare earth ions which are doped to the core of the rare earth-doped optical fiber 313, and the rare earth ions are in the excited state. The rare earth ions in the excited state emit spontaneous emission light of a specific wavelength. The spontaneous emittion light is amplified and propagated in the rare earth-doped optical fiber, and are output as an amplified spontaneous emission (ASE). Alternatively, the WDM coupler 312, the rare earth-doped optical fiber 313, the isolator 314, the output coupler 315 and the light switch element 317 are connected in a ring shape. The ASE passes through these portions and circulates, and is amplified again by the rare earth-doped optical fiber 313 until laser oscillation is achieved, and then part of this light is output as laser light via the output coupler 315. In addition, when the light switch element 317 is operated in a normal low loss state, CW oscillation is obtained and thus the laser light is outputted as a continuous light. Alternatively, when the light switch element 317 is operated to repeat periodically the low loss state and the high loss state, pulse oscillation is obtained and thus laser output in pulse form is obtained.
Alternatively, as the PA, an amplifier having the structure shown in FIG. 3 is generally used. FIG. 3 shows an exemplary structure of the MOPA type fiber laser. The reference numeral 100 shown in FIG. 3 corresponds to the MO, the reference numeral 200 corresponds to the PA, and laser light output from the MO 100 is amplified by the PA 200 via the interstage isolator 316 and output.
The PA 200 is composed of a plurality of pump light sources 201, a photocoupler 203, a rare earth-doped double-clad fiber 105, and an isolator 206. The pump light source 201, the photocoupler 203, the rare earth-doped double-clad fiber 105, and the isolator 206 may be the same as those used in the fiber laser 100. For example, as the photocoupler 203, the photocoupler which is disclosed in Patent Document 2 is employed. The photocoupler 203 includes a plurality of pumping ports 102 each configured from multi-mode optical fiber, and a signal port 202 composed of a single mode fiber, and further includes an emission port 104 which is integrally formed by fusing and extending the ports described above. The laser light emitted from the MO 100 is coupled to the signal port 202, and coupled to the core of the rare earth-doped double-clad fiber 105 via the photocoupler 203.
Alternatively, the pump light source 201 is connected to the pumping port 102. The pump light is coupled to a first clad of the rare earth-doped double-clad fiber 105 via the photocoupler 203. The pump light coupled to the first clad of the rare earth-doped double-clad fiber 105 is absorbed into rare earth ions which are doped in the core, and then, the population inversion and the induced emission is generated. Therefore, the laser light propagating in the core is amplified and output via the isolator 206.
In the case of the MOPA type shown in FIG. 3, while the signal light from the MO is not entered when the rare earth-doped double-clad fiber of the PA is pumped to reach a predetermined population inversion factor, self-oscillation or a phenomenon called parasitic oscillation is generated so that there may generate pulses with significantly high peak power. The population inversion factor generating self-oscillation or parasitic oscillation is determined by the reflectance of the incident side and the emission side of the rare earth-doped double-clad fiber. In some cases, the pulses caused by self-oscillation are emitted from the rare earth-doped optical fiber to the photocoupler. The pulses reach the pump light source or the MO, so that there may be a risk of damage thereon. In addition, even when the pulses emitted from the MO is entered in a cycle in which the PA does not oscillate by itself and the fiber laser is in a normal operating state, the reflected light from the outside of the PA output may induce oscillation while the pulses are being input. In general, the pump light source of the PA emits the pump light even between the oscillation of pulses, and the rare earth-doped double-clad fiber is in the pumping state. Therefore, the ASE light is emitted from both ends of the rare earth-doped double-clad fiber, and the reflected light may be coupled again to the fiber laser according to the surface state of an object. As a result, the reflected light acts as a source of the generation of oscillation, the pulses with significantly high peak power generated by self-oscillation are emitted from the rare earth-doped optical fiber to the photocoupler. These pulses reach the pump light source or the MO, so that there may be a risk of damage thereon.
In order to solve the problems, some solutions have been proposed.
As a first solution, the method as disclosed in Patent Document 3 is exemplified. According to the method, by inserting isolators on both ends of the rare earth-doped optical fiber, the reflectance is suppressed so as to be lower, so that self-oscillation is suppressed. Further, by providing a short wavelength pass filter at the emission side of the pump light source, repeated coupled to the rare earth-doped double-clad fiber of the ASE which has been emitted from the rare earth-doped double-clad fiber and is reflected by the pump laser, is suppressed. That is, the reflectance on the incident side and the emission side of the rare earth-doped fiber is suppressed as much as possible, so that self-oscillation is suppressed.
As a second solution, the method as disclosed in Patent Document 4 is an exemplary example. According to the method, the optical fiber amplifier is divided into two stages, and the isolator is provided between the stages. The gain of the front stage is suppressed so as to be lower, so that self-oscillation is suppressed. Even if the gain of the rear stage is high, since the ASE light is always coupled thereto, the ASE light is amplified but not to a sufficient level to cause self-oscillation.
As a third solution, the method as disclosed in Patent Document 5 is an exemplary example. In this method, the fiber laser having a structure such that the resonator is configured by providing fiber bragg gratings (FBG) at both ends of the rare earth-doped double-clad fiber, one of the FBG is connected to the multi-mode fiber, and the pump light from the pump light source is coupled to the rare earth-doped fiber via the multi-mode fiber. According to such a structure, since the diameter of the core of the multi-mode fiber is larger than that of the core of the rare earth-doped double-clad fiber, there is no reflection on the FBG, but unnecessary wavelength ASE which is coupled to the multi-mode fiber is reflected, so that the recombination ratio of the rare earth-doped optical fiber to the core is reduced. Therefore, self-oscillation can be suppressed. In addition, if self-oscillation occurs in some cases, since the generated pulses are firstly coupled to the multi-mode fiber, the above damage is suppressed because the spot diameter becomes larger even though the pump light is focused on the pump light source via the lens.    [Patent Document 1] Japanese Granted Patent, Publication No. 2977053    [Patent Document 2] Specification of U.S. Pat. No. 5,864,644    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H05-136498    [Patent Document 4] Japanese Granted Patent, Publication No. 2653936    [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. H10-56227